Cable television (CATV) equipment, such as a Remote PHY node, is powered by quasi-square-wave AC power (QSW), which is carried by the same coaxial cable that carries the RF signal delivered to and from subscribers in a cable television system. Older CATV systems were limited to 60V QSW, while newer CATV systems often support 90V QSW, which enables a higher power transfer to devices (such as nodes, CATV amplifiers, Wi-Fi hot spots, and the like) connected to the coax distribution network.
Both the AC and the RF signals often travel through a coax distribution network that is composed of a chain of RF taps that are connected together by a length of coaxial hardline cable. These RF taps are used to tap some of the RF signal carried by the hardline coaxial cable and provide the tapped RF signal to drop cables which convey the tapped RF signal to subscribers' homes.
Taps are occasionally opened and maintained by cable technicians for various maintenance, repair, and testing tasks. The construction of these taps is such that the RF electronic circuitry that perform the bulk of its functionality is physically contained on a removable “face plate” of the tap rather than affixed within its base chassis. This enables easy replacement of the tap's electronic circuitry when the need arises without a need to splice a new tap to the hardline coaxial cable.
However, opening and removing the face plate of these taps causes an interruption in the signal path that travels in and out of the tap face plate and then continues to travel through the tap chain, thereby disconnecting both the RF signal and the QSW AC power from the rest of the chain. To solve this potential problem when tap face plates are removed, the taps are designed with a “make-before-brake” alternative path in the tap chassis. The mechanical action of removing the tap face plate causes disengagement of the signal path from the RF electronic part included in the face plate and the replacement of the signal path with an alternative path inside the tap chassis to provide continuity of the RF signal and the QSW AC power to the rest of the cable distribution network. The term make-before-brake signified that, during the removal of the tap face plate, the alternative path is established before the main path is removed, allowing the RF signal and the QSW AC power to flow through the tap before, during and after the removal operation. Similarly, during the reinsertion of the tap face plate, the main path is established before the alternative path is removed, allowing the RF signal and the QSW AC power to flow through the tap before, during and after the reinsertion operation.
Unfortunately, quite often the make-before-brake mechanism in the tap does not operate as intended. This can happen when the tap face plate is removed while slightly askew, which causes one of the tap face plate's two connections (i.e., the in port or the out port) to disengage from the tap chassis before the other connection is disengaged. This sequential disengagement can cause a very short interruption, typically lasting in the order of several hundreds of milliseconds, in the RF signal and the QSW AC propagation through the tap chain. As a result, an active CATV device which is being powered by the QSW AC power propagating through the tap chain will experience a short interruption of power, and will often lose its ability to operate for a short period, corresponding to the length of interruption plus a recovery time once the QSW AC power is restored.
In a traditional analog cable distribution system (i.e., a cable distribution system existing prior to the use and inclusion of sophisticated CPU embedded signal processing elements in the outside plant portion of the system), such an interruption caused by removing a tap face plate would typically cause the delivery of cable services to customers to be interrupted for less than a second. The traditional analog cable system equipment that is installed at the customer premises is designed to “survive” such interruptions without having much, if any, lasting ramifications. A user may experience a very short “tiling” on his TV screen, an almost unnoticeable momentary slowdown in Internet access, or a very short brake in audio during a call using a cable connected telephone, but all these will disappear after a second or so.
However, when CPU-embedded signal processing devices, which may be installed on the cable distribution plant, experience a similar such split-second interruption in their QSW AC powering source, these CPU-embedded devices often require several minutes to recover from such a power interruption, which is far more noticeable to customers. Cable services delivered to customers through them will often suffer corresponding long periods of interruption even if the source power interruption was just several hundreds of milliseconds.
The line power supply that shapes and feeds the QSW power to the coax distribution network is designed to provide various protection and safety mechanisms. Among these protection mechanisms, the line power supply limits the maximum current that can be withdrawn by the cable plant. Specifically, when the amount of current withdrawn is too high, the line power supply can reduce the QSW voltage. In more extreme cases of high current withdraw lasting several seconds (such as when a short circuit is applied somewhere to the coax), the line power supply may remove its output voltage completely from the coax distribution network, which typically results in the loss of all cable services provided through the coax distribution network.
Since the loss of services is a severe adverse effect, the line power supply is required by design to try and resume the delivery of power to the coax distribution network. This is done to enable the recovery of CATV devices when the issue that caused the high current withdraw was only temporary and is no longer present, and to prevent the need for a manual maintenance and repair action that may take a substantial time to commence (typically an hour or more). The line power supply is typically designed to try to resume normal operation every configurable length of time that can range over several seconds to several minutes.
Thus, a technician working on a part of the coax distribution network has the potential to cause a momentary electrical short, which, if lasting less than a few seconds, will typically cause loss of QSW AC power for a similar time, while a short circuit that lasts longer will typically cause the line power supply to protect the CATV system from excessive current by removing the QSW power from the coax distribution network for the configured length of time. When the coax distribution network is driven by traditional HFC equipment and nodes, the resulting service interruption to subscribers is often limited to the same several seconds or minutes as well, although some customer premises equipment (CPE), such as cable modems, will be required to register to the cable operators network again after cable signals are resumed, resulting in a loss of service that may last several minutes more than the power interruption itself.
Advanced systems, such as the CableLabs specified Remote-PHY system, offer many advantages to cable operators. Unfortunately, the R-PHY system introduces some disadvantages as well. One such disadvantage is the typical long restart process the R-PHY device (RPD) has to undergo when powered on, which includes a software boot process, a network authentication process, obtaining an Internet Protocol (IP) address, a timing synchronization process, registering with the R-PHY system, a configuration process, and turning on services. As a result, even a short split second power interruption which causes a similar split second interruption of cable services in a traditional hybrid fiber-coaxial (HFC) cable network can cause as much as 10 minutes of cable services interruption when these services are delivered by an RPD or an R-MACPHY device (RMD).